fatal error: X11/Xlib.h:

hi. i am using ubuntu 12.04 and trying to install NS2.35
for that matter i downloaded ns-allinone-2.35, unzipped it, started installation, the installation went well until it gave an error message and stopped. the error message is as follows:

one more thing is that while doing 'sudo apt-get update' it gives me the failure installing packages of X because of unreachable site, it gives me Err http://linux.ntuoss.org precise/universe amd64 Packages

@dodo3773! may be what you suggested is the right place from where i can get amd updation, but i really have no idea about how to do it. need help